I. Field of the Invention
This invention relates to characterization of misting properties of fluids. In particular, this invention relates to methods and apparatus for measuring a misting property of metalworking fluids.
II. Description of Related Art
Many industrial processes make use of fluids for lubricating and cooling, particularly machining processes, i.e. processes involving mechanical cutting or forming of workpieces. Machining processes include cutting processes such as sawing, drilling, milling, grinding, turning, broaching, reaming, tapping, and planing, and forming processes such as bending, ironing, punching, rolling, stamping, die cutting, and forging. When lubricating and cooling fluids are applied as streams to the interface between the workpiece and tool, mists can be generated as a function of energy input to the fluids by the mechanics of the machining process. Such generation of mists is most common in processes in which the fluid stream contacts the periphery of a rotating member, whether a tool, as in drilling, milling, tapping, reaming, or boring, or a workpiece as in turning or some types of grinding. In such processes, sufficient shear stresses are developed by contact of the fluid with the rotating member to cause separation of droplets from the fluid stream, and to propel the droplets into the surrounding atmosphere. Such mist generation is particularly prevalent in grinding operations by virtue of the relatively high surface speeds at which grinding is performed, hence high energy input to the fluid, the relatively high volume of fluid applied, hence large quantities of fluid to be converted to droplets, and the rough surface of grinding wheels tending to capture fluids at the surface and hence increase shear stresses imparted to fluids.
The nature of mist generation in industrial processes is such that a broad spectrum of droplet masses is produced. Further, droplets are propelled from these processes in varying directions and are, consequently, subject to collisions with other droplets, altering droplet masses and energies. In general, these processes produce droplets which are subject to different transport effects. In particular, transport of droplets of relatively large mass will be dominated by inertia and gravity, while transport of droplets of relatively small mass will be dominated by Brownian motion. The droplets of smaller mass are known to contribute to the persistence of mists long after mist generation has ceased. As the transport of fluid droplets and the persistence of mists contribute to potential human exposure in industrial operations, and the concentration of mists in industrial environments is subject to regulation as relating to health and safety in the workplace. Of particular concern in this regard are droplets small enough to be respirable, these smaller droplets being susceptible of transportation farthest from the source. Regulations set limits on the concentration of mist in the workplace at locations proximate to industrial equipment where production personnel can be expected to be exposed to such mists. In addition to concerns of exposure of personnel, generation of mists from machining fluids has the attendant disadvantages of increasing loss of fluid, with potential loss of functionality as fluid composition is changed, and deposition on surfaces of facilities and equipment of contaminants carried in such mists.
In light of the adverse consequences of mist generation, and particularly in light of the regulation of mist concentration in the workplace, it is desirable to characterize the misting properties of machining fluids. In particular, it is desirable to characterize the influence on misting properties of fluid constituents to identify fluids and constituents which have the potential for reducing mist generation while maintaining desirable functions of the fluids. Hence, methods and apparatus for measuring misting properties of machining fluids in industrial processes are needed.
It is known to measure mist generation in production environments, however it is clearly impractical to dedicate use of production machinery to measurements required for characterization of misting properties of the spectrum of machining fluids to be encountered in industrial operations. Besides the obvious high cost of equipment utilization, the use of such machines increases the difficulty of controlling conditions relevant to characterization of fluid properties. Hence, methods and apparatus are needed for characterization of misting properties of aqueous fluids under controlled conditions which simulate an industrial process while permitting elimination of variables difficult to document in a production environment.
Known laboratory techniques for controlled generation of mists generally lack one or more characteristics of machining processes which influence generation of mists in industrial production. For example, it is known to use atomizers to produce mists from a reservoir of the fluid under test. Atomizers, typically single point sources, do not generally produce random motion of fluid droplets characteristic of machining processes. Further, while production equipment recirculates machining fluids through the machining process, hence recombines mist droplets collected on machine surfaces with other fluid from a reservoir, laboratory equipment known for studying mists does not provide for collection of mist, its recombination with fluid used for mist generation and recirculation of the resulting fluid.